feat: HTML sitemap groups API

- GET /_sitemap/api/v1/html?locale= answers the overview: the listed
  addresses of the XML sitemaps by group, in the order pages, offers,
  services, categories (as a tree), posts (newest first), tags, authors,
  other content types, each with its title in the locale, total, the
  first html_page_size links and more
- GET /_sitemap/api/v1/html/{group}?locale=&page= answers one page of a
  group; past the last page, an unknown group or the HTML sitemap off
  answer 404; every answer is noindex; per-IP bucket sitemap-html
- the HTML sitemap tab: html_enabled, the address per locale (html_paths)
  and html_page_size 50 to 500, with errors.html_path and errors.page_size
- contract.Listing.Settings answers HTMLEnabled, HTMLPaths, HTMLPageSize
  and KeyFile

classes/settings.go

@@ -1,6 +1,7 @@
 package classes
 
 import (
+	"encoding/json"
 	"strings"
 
 	"git.golem15.com/golem15/sm-sitemap-plugin/models"
@@ -59,3 +60,39 @@ func Absolute(site, u string) string {
 	}
 	return ""
 }
+
+// MergeHTMLPaths merges submitted HTML sitemap addresses (locale to path)
+// into the stored ones: a sent locale replaces its address, a blank one
+// removes it, the others keep theirs. problem is true when a sent address
+// is not a models.ValidHTMLPath; nothing is merged then.
+func MergeHTMLPaths(db *gorm.DB, sent map[string]string) (map[string]string, bool) {
+	settings, err := LoadSettings(db)
+	if err != nil {
+		return nil, true
+	}
+	merged := settings.EffectiveHTMLPaths()
+	for locale, path := range sent {
+		locale, path = strings.TrimSpace(locale), strings.TrimSpace(path)
+		if locale == "" {
+			continue
+		}
+		if path == "" {
+			delete(merged, locale)
+			continue
+		}
+		if !models.ValidHTMLPath(path) {
+			return nil, true
+		}
+		merged[locale] = path
+	}
+	return merged, false
+}
+
+// StoreHTMLPaths writes the HTML sitemap addresses to settings row 1.
+func StoreHTMLPaths(db *gorm.DB, paths map[string]string) error {
+	raw, err := json.Marshal(paths)
+	if err != nil {
+		return err
+	}
+	return db.Exec(`UPDATE golem15_sitemap_settings SET html_paths = ?::jsonb WHERE id = 1`, string(raw)).Error
+}